1. Field of the Invention
The present invention relates to a metal cord for reinforcing rubber products such as tires and belts.
2. Description of the Prior Art
Metal cords such as steel cords have been used to reinforce rubber products such as tires and belts. In recent years, in order to improve durability and anti-fatigue properties of cords under humid, corrosive circumstances, a so-called open type cord is popular. This cord is obtained by loosely twisting a plurality of wires a, as shown in FIG. 1. In a conventional open type steel cord, rubber permeates into the cord through spaces between adjacent wires a, and moisture, salt water or the like does not enter into the cord. As a result, durability and anti-fatigue properties of the cord are improved. In a cord of this type, a preforming ratio of each wire normally falls within the range of 1.05 to 1.40, and wires a are preformed almost identically.
The preforming ratio as used in the specification and the appended claims means to form wire members constituting a cord in a spiral shape having the same pitch and diameter (Dc) as those of the finished cord and is given by a quotient (Df/Dc) obtained by dividing spiral diameter Df by diameter Dc of the close packed cord. The preforming ratio of wire members in an open type cord exceeds 1.
According to the test results of the present inventors, the diameter of each wire in a conventional steel cord is as small as about 0.15 to 0.38 mm. When tension acts on the cord, the spiral pattern preformed in each wire is elongated along the direction of tension. Upon an increase in tension force, the distance between the adjacent wires is decreased. In the worst case, it is difficult to allow permeation of rubber into the cord. The critical tension force causing the state described above varies according to the constitutions and wire diameters of the cords. In steel cords disclosed in Japanese Patent Disclosure (Kokai) No. 55-90692 (corresponding to U.S. Pat. No. 4,258,543) and Japanese Utility Model Publication No. 58-48392 (corresponding to U.S. Pat. No. 4,333,306), the critical tension force is about 20 to 25 N (Newton). Such a critical tension force greatly degrades rubber permeability of the cord.
At the time of embedding of steel cords into rubber in the industrial fabrication of tires, a predetermined tension force is applied to cords when the cords are aligned or vulcanized. The tension force normlly falls within the range of 15 to 25 N. In a conventional open type steel cord, the tension force acting upon embedding of the cord in rubber is substantially equal to the critical tension force greatly influencing rubber permeability. For this reason, rubber permeability is degraded. As a result, improvements in durability and anti-fatigue properties of cords under the humid, corrosive circumstances cannot be expected.
Japanese Patent Disclosure (Kokai) No. 57-43866 (corresponding to U.S. Pat. No. 4,399,853) discloses a steel cord of an open type wherein elongation of the cord is kept within a predetermined range to prevent distortion of spiral pattern of the cord. However, in such a cord, since changes in elongation of the cord under the tension of 20 to 50 N were excessively large, errors occurred in the dimensions of cords during vulcanization. In addition, the modulus of a composite body of wires and rubber was undesirably changed.